Increase Of Electron-phonon Coupling Research Articles

Raman spectroscopy of a few layers of bismuth telluride nanoplatelets.

We can shape the electronic and phonon properties of Bi2Te3 crystals via the variation of the number of layers. Here, we report a Raman study with the aid of first-principles calculations on few-layered Bi2Te3 systems ranging from 5 to 24 nm layer thickness using 1.92, 2.41 and 2.54 eV excitation energies. We examine how the frequency position, intensity and lineshape of the main Raman modes (A11g, E2g, and A21g) behave by the variation of the layer thickness and excitation energy. We observed a frequency dispersion on the number of layers of the main modes, indicating changes in the inter- and intra-layers interaction. A resonant Raman condition is reached for all modes for samples with 11 and 18 nm thickness because of van Hove singularities at the electronic density of states. Also, the Breit-Wigner-Fano line shape of the A21g mode shows an increase of electron-phonon coupling for thick layers. These results suggest a relevant influence of numbers of layers on the Raman scattering mechanics in Bi2Te3 systems.

Rapid Decoherence Induced by Light Expansion Suppresses Charge Recombination in Mixed Cation Perovskites: Time-Domain ab Initio Analysis.

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we have investigated the effect of light-induced lattice expansion on the nonradiative electron-hole recombination in the mixed-cation perovskite FA0.75MA0.25PbI3. We demonstrate that charge carrier lifetime extends by a factor of 1.5 within 1% lattice expansion; the bandgap grows only by 0.04 eV; the electron-phonon coupling increases by 13%; and the decoherence time shortens by 37%. The small bandgap change has negligible influence on recombination times. Lattice expansion enhances atomic fluctuations that lead to the enhancement of electron-phonon coupling and acceleration of decoherence. By creating several high-frequency phonon modes, the lattice expansion shortens the decoherence time further. As a result, rapid decoherence beats an enhanced electron-phonon coupling, playing the dominant role in suppressing the nonradiative electron-hole recombination. The light-induced lattice expansion or strain effects provide a rational route to improve the perovskite photovoltaic and photoelectronic device performance.

Ion tracks formation through synergistic energy processes in strontium titanate under swift heavy ion irradiation: Experimental and theoretical approaches

The primary motivation for studying ion-solid interactions and the responses of various materials to energetic irradiation is the fundamental necessity and importance to get a deeper understanding of how irradiation changes the lattice structures and physico-chemical properties of solid materials. Under the action of extreme electronic energy deposition induced by swift Kr17+, Ar12+ and Ni19+ irradiation, the formation mechanism of latent tracks with different damage morphologies in pristine and predamaged SrTiO3 crystals are analyzed utilizing experimental characterizations of the lattice damage and numerical calculations from the inelastic thermal spike model. In contrast to Ar12+ and Ni19+ irradiation, Kr17+ irradiation, with an ion energy of 2.35 MeV/u and electronic energy loss of 16.7 keV/nm, increased the lattice temperature of the pristine crystal to 3500 K, and via a subsequent rapid quenching process, discontinuous tracks including individual spherical defects and discontinuous cylindrical damaged zones were formed. The related damage morphology demonstrates the threshold value of the lattice-temperature change induced by irradiation for track production in pristine SrTiO3, which is fundamental and essential for comparing irradiating-ion velocities and electronic energy losses. Owing to the decrease in thermal conductivities and increase in electron-phonon coupling in Au+-irradiation-damaged SrTiO3, subsequent Si3+, Ar12+ and Kr17+ irradiation significantly increased the lattice temperature of predamaged crystals to greater than 3500 K, leading to discontinuous and continuous track production in the predamaged crystals. Thus, the pre-existing damage produced by nuclear energy loss interacted synergistically with the electronic energy loss and effectively enhanced the lattice-temperature increase and promoted track formation.

Mobility of two-dimensional materials from first principles in an accurate and automated framework

We present a first-principles approach to compute the transport properties of 2D materials in an accurate and automated framework. We use density-functional perturbation theory in the appropriate bidimensional setup with open-boundary conditions in the third direction. The materials are charged by field effect via planar counter-charges. In this approach, we obtain electron-phonon matrix elements in which dimensionality and doping effects are inherently accounted for, without the need for post-processing corrections. This treatment highlights some unexpected consequences, such as an increase of electron-phonon coupling with doping in transition-metal dichalcogenides.We use symmetries extensively and identify pockets of relevant electronic states to minimize the number of electron-phonon interactions to compute; the integrodifferential Boltzmann transport equation is then linearized and solved beyond the relaxation-time approximation. We apply the entire protocol to a set of much studied materials with diverse electronic and vibrational band structures: electron-doped MoS2, WS2, WSe2, phosphorene, arsenene, and hole-doped phosphorene. Among these, hole-doped phosphorene is found to have the highest mobility, with a room temperature value around 600 cm^2/(Vs). Last, we identify the factors that affect most phonon-limited mobilities, such as the number and the anisotropy of electron and hole pockets, to provide a broader understanding of the driving forces behind high mobilities in two-dimensional materials.

Influence of electron-phonon interaction on quantum phase transition in the triple quantum dots coupled to pseudo-gap Fermi system

We investigate the triple quantum dots with triangular geometry, in which one of the dots (embedded dot) is coupled to pseudo-gap Fermi system and electrons in the dot interact with a local phonon mode. The model, which we treat using the analytical argument and the numerical renormalization group, exhibits the quantum phase transition between ferromagnetically-coupled local moment phase and antiferromagnetically-coupled strong coupling phase depending upon electron-phonon coupling. We restrict our discussion to the case that inter-dot exchange couplings are much smaller than the Kondo temperature for the embedded dot. Since Fermi system has the density of states dependent on energy in power-law fashion, the embedded dot retains an unscreened spin degree of freedom even in Kondo regime. For the coupling to such Fermi system, the simultaneous increase of electron-phonon coupling and pseudo-gap exponent has the contrary effects on the quantum phase transition depending upon inter-dot couplings.

Influence of electron-phonon interaction on quantum phase transition in a triangular triple quantum dot

We investigate the quantum phase transition in triple quantum dot system with triangular geometry, in which one of the dots is connected to metallic leads and electrons in the dot interact with a local phonon mode. The influence of electron-phonon interaction on the quantum phase transition between local moment phase and Kondo screened strong coupling phase at the particle-hole symmetric point is studied based on the analytical arguments and the numerical renormalization group method. The results show that the critical value of tunnel-coupling between side dots decreases with the increase of electron-phonon coupling in “spin Kondo” regime. Furthermore, at a certain critical value of electron-phonon coupling, there appears only strong coupling phase, irrespective of tunnel-coupling between dots. The study of the influence of electron-phonon interaction on the quantum phase transition in triple quantum dot has the great importance for clarifying the mechanism of Kondo screening in the system.

Increase of self-heating effects in nanodevices induced by surface roughness: A full-quantum study

We present a full-quantum approach to investigate self-heating effects in nanoelectronic devices and exploit it to simulate rough nanowire field-effect transistors. Self-heating is found to significantly contribute (up to about 16%) to the degradation of the transistor performances, with an impact that is stronger for stronger roughness. The mechanism at the origin of the enhanced backscattering is the temperature increase due to the thermal conductivity reduction and the consequent increase of electron-phonon coupling.

Spin-resolved electron-phonon coupling in FeSe and KFe2Se2

The effect of the static magnetic moments of iron on electron-phonon interactions in layered FeSe and KFe${}_{2}$Se${}_{2}$ is studied. First-principles techniques based on the pseudopotential density functional approach and the local spin density approximation are utilized to calculate the band structures, phonon dispersions, and electron-phonon coupling properties. Our results indicate that the introduction of iron magnetic moments leads to significant changes in electronic structure induced by Fe 3$d$ states near the Fermi level, to phonon frequency softening for several vibrational modes, and to a dramatic increase in electron-phonon coupling for specific modes. The increase in Brillouin-zone-averaged coupling is about twofold. Our estimates of superconducting transition temperatures based on the McMillan equation yield values closer to experimental results for the spin-resolved case. However, these values are not large enough to explain the observed transition temperature.

Optical conductivity of polarons: Double phonon cloudconcept verified by diagrammatic Monte Carlo simulations

We study theoretically a fundamental issue in solids: the evolution of the optical spectra of polaron as the electron-phonon coupling increases. By comparing the exact results obtained by diagrammatic Monte Carlo method and the data got through exact diagonalization within an appropriate subspace of the phononic wavefunctions, the physical nature of the crossover from weak to strong coupling is revealed. The optical spectra are well understood by the quantum mechanical superposition of states with light and heavy phonon clouds corresponding to large and small polarons, respectively. It is also found that the strong coupling Franck-Condon regime is not accessible at realistic values of the coupling constant.

Quantum transport of double quantum dots coupled to an oscillator in arbitrary strong coupling regime

In this paper, we investigate the quantum transport of a double quantum dot coupled with a nanomechanical resonator at arbitrary strong electron-phonon coupling regimes. We employ the generalized quantum master equation to study full counting statistics of currents. We demonstrate the coherent phonon states method can be applied to decouple the electron-phonon interaction non-perturbatively. With the help of this non-perturbative treatment of electron-phonon couplings, we find that the phonon-assisted resonant tunneling emerges when the excess energy from the left quantum dot to the right one can excite integer number of phonons and multi-phonon excitations can enhance the transport in strong electron-phonon coupling regime. Moreover, we find that as the electron-phonon coupling increases, it first plays a constructive role to assist the transport, and then plays the role of scattering and strongly represses the transport.

Charge transport in DNA: Dependence of diffusion coefficient on temperature and electron-phonon coupling constant

The diffusion coefficient is calculated for a charge propagating along a double-stranded DNA, while it interacts with the nonlinear fluctuational openings of base pairs. The latter structural dynamics of DNA is described by the Peyrard-Bishop-Dauxois model [T. Dauxois, M. Peyrard, and A. R. Bishop, Phys Rev. E 47 R44 (1993)], which represents essential anharmonicities of base-pair stretchings. The dependence of the diffusion coefficient on the temperature and the electron-phonon coupling constant is presented. The diffusion coefficient decreases when either the temperature or the electron-phonon coupling increases. Analytical expressions are provided that describe the temperature dependence of the diffusion coefficient. The variation of the parameters of these expressions with the electron-phonon coupling constant is also discussed. These results quantitatively demonstrate how DNA structural nonlinear dynamics affects macroscopic charge transport properties.

Superconductivity in the Rh-based Heusler familyMRh2Sn

Superconductivity and structural characterization is reported for the Heusler structure compounds ${\text{ScRh}}_{2}\text{Sn}$, ${\text{LuRh}}_{2}\text{Sn}$, and ${\text{YRh}}_{2}\text{Sn}$. The superconducting ${T}_{\text{c}}$'s are 2.0 K, 2.9 K, and 4.1 K, respectively. The electronic contributions to the specific heat and the Debye temperatures are reported, and suggest that the observed trend in ${T}_{\text{c}}$ is governed primarily by an increase in electron-phonon coupling on going from ${\text{ScRh}}_{2}\text{Sn}$ to ${\text{YRh}}_{2}\text{Sn}$. The electron-phonon coupling constant ${\ensuremath{\lambda}}_{\text{ep}}$ and the normalized specific-heat jump $\ensuremath{\Delta}C/\ensuremath{\gamma}{T}_{\text{c}}$ suggest that the $M{\text{Rh}}_{2}\text{Sn}$ system evolves from weak coupling to moderate coupling superconductivity.

Linear response study of strong electron-phonon coupling in yttrium under pressure

Linear response methods are applied to identify the increase in electron-phonon coupling in elemental yttrium that is responsible for its high superconducting critical temperature ${T}_{c}$, which reaches nearly $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $115\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. While the evolution of the band structure and density of states is smooth and seemingly modest, there is strong increase in the $4d$ content of the occupied conduction states under pressure. We find that the transverse mode near the $L$ point of the fcc Brillouin zone, already soft at ambient pressure, becomes unstable (in harmonic approximation) at a relative volume $V∕{V}_{0}=0.60$ $(P\ensuremath{\approx}42\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$. The coupling to transverse branches is relatively strong at all high-symmetry zone boundary points $X$, $K$, and $L$. Coupling to the longitudinal branches is not as strong, but extends over more regions of the Brillouin zone and involves higher frequencies. Evaluation of the electron-phonon spectral function ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$ shows a very strong increase with pressure of coupling in the $2--8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ range, with a steady increase also in the $8--20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ range. These results demonstrates strong electron-phonon coupling in this system that can account for the observed range of ${T}_{c}$.

Low-energy excitations inLa1.2Sr1.8Mn2O7investigated by ellipsometry

The low-energy excitations of the bilayered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ have been explored by spectral ellipsometry from two faces of a single crystal over the range from 0.006 to 0.6 eV. This compound is a paramagnetic insulator at ambient temperature, with a transition to a ferromagnetic metal below a Curie temperature $({T}_{\mathrm{c}})$ of 125 K. Both the $ab$-plane and $c$-axis temperature-dependent conductivities have been determined. Essentially no temperature-dependent behavior is observed above ${T}_{\mathrm{c}}$ although below ${T}_{\mathrm{c}}$ both the phonon and electronic contributions are strongly temperature sensitive. The highest-frequency phonons, especially those involving Mn-O bond stretching, split and show frequency changes consistent with structural results in the literature, and furthermore there is clear evidence of an increase in electron-phonon coupling at and below ${T}_{\mathrm{c}}$. We interpret the temperature-dependent electronic spectral contribution in the light of recent calculations that indicate that a mixed phase exists in the doped manganites below ${T}_{\mathrm{c}}$, with coexisting regions of an itinerant large-polaron phase and a localized small-polaron phase.

Electron–phonon coupling of Cr 3+ in YAG and YGG

The electron–phonon (EP) coupling of the 2 E→ 4 A 2 -emission from Cr 3+ has been measured for the aluminum garnet Y 3Al 5O 12 (YAG) and the gallium garnet Y 3Ga 5O 12 (YGG) using three different methods; determination of the vibronic transition probability A vib from the decay time τ and the ratio between the vibronic- and zero-phonon line intensity (1), temperature-dependent line-broadening resulting in a strength parameter α ̄ (2) and temperature-dependent line shift measurements, resulting in an electron–phonon coupling strength parameter α (3). All three methods show a stronger EP coupling for the gallium garnet YGG compared to the aluminium garnet YAG. This increase in EP coupling is explained by an increase in covalency in the gallium garnet compared to the aluminium garnet, which results in larger overlap of electron wave functions of the transition metal ion Cr 3+ with the ligands. Also lowering of the 4 T 2 state going from Cr 3+-doped YAG to YGG could give a contribution to the increase of EP coupling for YGG : Cr 3+.

Polaronic effects in strongly coupled electron-phonon systems: Exact diagonalization results for the 2D Holstein t- J model

Ground-state and dynamical properties of the 2D Holstein t- J model are examined by means of direct Lanczos diagonalization, using a truncation method of the phononic Hilbert space. The single-hole spectral function shows the formation of a narrow hole-polaron band as the electron-phonon coupling increases, where the polaronic band collapse is favoured by strong Coulomb correlations. In the two-hole sector, the hole-hole correlations unambiguously indicate the existence of inter-site bipolaronic states. At quarter-filling, a polaronic superlattice is formed in the adiabatic strong-coupling regime.

Dynamics of self-trapping on a dimer with anharmonic coupling

The dynamics of electron self-trapping on a dimer is studied with Poincar\'e maps. Formation of two symmetric trapping zones consisting of resonant tori is shown to occur as the electron-phonon coupling increases. When the vibrational anharmonic coupling is sufficiently strong, we observe a symmetry breaking caused by the induced stochasticity that destroys all the tori of one trapping zone while leaving those of the other zone unaffected. Chaotic and regular motion, thus, simultaneously exist on different sites, trapping the electron only on the site with periodic motion.

Diamagnetic susceptibility of a Fröhlich polaron

Numerical calculations of the diamagnetic susceptibility of a Fröhlich polaron are presented on the basis of Feynman's path-integral method with the generalized trial action developed earlier. The results show that the susceptibility deviates from the free-electron value and becomes practically temperature independent as the electron-phonon coupling increases.